Variable displacement hydraulic pumps are used in a variety of applications. For example, hydraulic construction machines, earthworking machines, and the like, often use variable displacement hydraulic pumps to provide the pressurized hydraulic fluid flow required to perform desired work functions. Operation of the pumps can be subject to variations in pressure and flow output caused by variations in load requirements. It has long been desired to maintain the pressure output of the pumps in a consistent manner so that operation of the hydraulic systems is well behaved and predictable. Therefore, attempts have been made to monitor the pressure output of a pump, and control pump operation accordingly to compensate for changes in loading. The pressure output of a variable displacement hydraulic pump is typically adjusted by changing the angle of a swashplate of the pump which changes the displacement of the pump.
As the power requirements of the machines in which variable displacement hydraulic pumps increases, so does the hydraulic fluid power requirements. However, due to restrictions in the structure of variable displacement hydraulic pumps as well as the properties of the hydraulic fluid, the size of the pumps that can be used practically is limited. Thus, in applications where a large flow of pressurized hydraulic fluid is desired, two variable displacement pumps can be arranged with their discharge ports connected together. Since the pump discharge ports are connected, the two pumps are hydraulically interacting with each other. In particular, the discharge pressures of the two pumps are always the same. Consequently, with a given pump load flow, any change in either swashplate angle will affect the discharge pressure. If the swashplates of the two pumps are controlled independently by respective control valves with feedback of the swashplate positions and the pump discharge pressure, several closed loops are formed making the design of the control system significantly more complicated than for a single pump.
One issue with controlling two pumps with a connected discharge port can occur in circumstances where the parameters of the two pumps are different either due to manufacturing and assembly tolerances but the two controllers are identical or where the parameters of the two controllers are different. Under such circumstances, the swashplate angles for the two pumps will be different under steady state conditions. As a result, a positive feedback process in the control systems can lead to the positions of the swashplates being moved to opposite extremes. In particular, a pressure increase in the system can cause the controller of the first pump to move the pump swashplate into a position that decreases the displacement of the pump (i.e., de-stroke the swashplate). This decreases the pressure in the system. However, if the pressure decreases too much, the controller for the second pump may attempt to increase the pressure in the system by directing the swashplate of the second pump into a position that increases the displacement of the second pump (i.e., up-stroke the swashplate). If the pressure increases too far, the controller for the first pump will attempt to compensate by decreasing the pressure by again decreasing the displacement of the first pump. Again, if the reduction goes too far, the second controller will yet again increase the displacement of the second pump to increase the pressure in the system. This loop can continue until the swashplate of one of the pumps hits its extreme position, i.e. fully up-stroked or full de-stroked. When the swashplate is in such a position, the performance and stability of the hydraulic system can be significantly degraded.
Another issue with controlling two pumps with a connected discharge port can arise when trying to optimize the operating efficiency of the two pumps. With a given set of other working conditions, the efficiency of the two pumps depends on their displacement. Thus, optimizing the operating efficiency of the hydraulic system for a particular discharge pressure, involves selecting the most favorable combination of the displacements of the two pumps. However, since the discharge ports of the two pumps are connected, any change in the displacement of one pump will necessarily affect the other one. As a result, a system that controls the two pumps with the same command cannot optimize the efficiency of the two pumps.